Final answer:
In season 2, Jet finds out that Iroh and Zuko are firebenders when he sees them heating tea using firebending in the city of Ba Sing Se. His suspicions are confirmed when he witnesses Zuko bending fire during a confrontation with Azula.
Step-by-step explanation:
In season 2 of the animated series Avatar: The Last Airbender, Jet discovers that Iroh and Zuko are firebenders while they are staying in the Earth Kingdom city of Ba Sing Se.
In the episode "The Drill," Jet and his Freedom Fighters spot Iroh and Zuko heating tea using firebending. Jet confronts them, accusing them of being firebenders. When they try to deny it, Jet mentions that he saw the fire and urges them to leave the city.
Jet's suspicion is confirmed in the episode "City of Walls and Secrets" when he witnesses Zuko bending fire during a confrontation with Azula.